[SCSI] remove flush_scheduled_work() usages

Simple conversions to drop flush_scheduled_work() usages in
drivers/scsi.  More involved ones will be done in separate patches.

* NCR5380, megaraid_sas: cancel_delayed_work() +
  flush_scheduled_work() -> cancel_delayed_work_sync().

* mpt2sas_scsih: drop unnecessary flush_scheduled_work().

* arcmsr_hba, ipr, pmcraid: flush the used work explicitly instead of
  using flush_scheduled_work().

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>